As an example of an industrial process that utilizes rappers, the exhaust of smokestack type plants, such as coal-fired power plants, cement kilns and pulp mills, is cleaned by electrostatic precipitators with vertical plates to remove particulate matter and to thus mitigate air pollution. In the evacuation of the ash residue it is common practice to prevent adhesion, congestion, buildup and flow stoppage by rapping such elements as precipitator plates, high voltage wires and electrodes, and other material-handling structure in a manner to shake the unwanted particles loose.